Seamless steel pipes are used in the drilling of oil and gas wells of crude oil, natural gas and the like (hereinafter, these are collectively referred to simply as “oil wells”) or used in transmission etc. thereof. In recent years, in association with the development of oil wells in deeper seas, the performance required for seamless steel pipes used as transmission pipes has become rigorous, while demand for heavy-wall seamless steel pipes has been increasing. In general, when the wall thickness of seamless steel pipes increases, it becomes difficult to ensure toughness, let alone to obtain high strength.
[Method of Manufacturing Seamless Steel Pipe]
A seamless steel pipe can be manufactured, for example, by the Mannesmann process. This process includes the steps of:    (1) piercing-rolling a billet by a piercing-rolling mill (a piercer) to form a blank pipe (hereinafter, referred to as a “hollow shell”);    (2) elongation-rolling the hollow shell by an elongation-rolling mill (for example, a mandrel mill);    (3) sizing the elongated hollow shell by a sizing mill (for example, a sizer); and    (4) ensuring strength and toughness in the steel pipe by heat treatment (for example, quenching step followed by tempering step).[Prior Art]
It is well known that in general, when steel products are subjected to thermal refining, grains become refined and toughness is improved. Therefore, in order to secure strength and toughness in heavy-wall seamless steel pipes, the heat treatment of (4) above is particularly important.
In thermal refining, in general, an atmosphere temperature control type heating furnace is used which heats materials to be heat treated by the atmosphere heated in the furnace by use of burners. However, when thermal refining by an atmosphere temperature control type heating furnace is applied to steel pipes with heavy wall thickness (hereinafter, referred to as heavy-wall steel pipes), an expected level of grain refinement may not be achieved in some cases, with the result that the toughness of the steel pipes decreases.
On the other hand, the following heat treatment methods have been disclosed as techniques for carrying out the heat treatment of steel pipes by induction heating in order to ensure toughness.
Patent Literature 1 proposes a heat treatment method which involves heating a steel pipe, which is obtained by rolling and forming a billet in heated condition, by induction heating from temperatures of not more than 500° C. to the temperature range from Ac3 point to 1000° C. and quenching the steel pipe, and thereafter performing tempering in the temperature range from 450° C. to Ac1 point, thereby improving the low-temperature toughness of the steel pipe.
Patent Literature 2 proposes a heat treatment method which involves induction heating a steel pipe, which is obtained by rolling and forming a billet in heated condition, to temperatures in the range of 850° C. to 1050° C., and quenching the steel pipe by performing water cooling at cooling rates of not less than 100° C./s from austenite territory, thereby improving the toughness of the steel pipe.
Although Patent Literatures 1 and 2 above propose the use of induction heating in quenching, these do not propose the frequency or heating rate of induction heating, and are not directed to a heavy-wall seamless steel pipe. Therefore, even when the heat treatment methods proposed in Patent Literatures 1 and 2 are applied to the heat treatment of a heavy-wall seamless steel pipe, it is impossible to sufficiently ensure the toughness of a steel pipe to be obtained.
Patent Literature 3 proposes a method by which, in the heat treatment of a clad steel pipe consisting of dissimilar metals, one in the outer circumference and the other in the inner circumference of the steel pipe, induction heating is performed by using a coil which is arranged surrounding the outer surface of the steel pipe and heats an outer circumference side metal and a coil which is inserted into the bore of the steel pipe and heats an inner circumference side metal, thereby heating the outer circumference side and inner circumference side of the steel pipe, respectively, at a different temperature. The frequency of induction heating disclosed in Patent Literature 3 is in the range of 1000 Hz to 3000 Hz.
Patent Literature 4 proposes a heating method in an in-line heat treatment of a seam portion of an ERW steel pipe. This heating method includes the steps of:    (1) heating a seam portion to a temperature in the range of 600° C. to 700° C. by induction heating at a frequency in the range of 700 Hz to 3000 Hz;    (2) subsequently, heating the seam portion to a temperature in the range of 700° C. to 750° C. by induction heating at a frequency in the range of 700 Hz to 800 Hz; and    (3) lastly, heating the seam portion to a temperature in the range of 900° C. to 1050° C. by induction heating at a frequency in the range of 700 Hz to 3000 Hz.
It is stated that by using the induction heating (1) to (3) above, while keeping lower the temperature on the inner circumference side of the seam portion, the outer circumference side is heated to a prescribed temperature, and the input energy is reduced.
Patent Literature 5 proposes a method by which, in the manufacture of an ERW steel pipe, after the whole steel pipe is heated by induction heating at a frequency of less than 100 kHz in forming the seam portion by a solid-phase pressure welding, the welded portion of the steel pipe is heated by induction heating at a frequency of not less than 100 kHz.
Patent Literatures 3 to 5 above disclose methods of heating a steel pipe by induction heating, and induction heating at a high frequency in the range of 700 Hz to 100 kHz is used. All the Patent Literatures 3 to 5 describe methods aimed at heating the surface of a steel pipe or along with the substrate to a specific depth, and even if these methods are applied to the heat treatment of a heavy-wall seamless steel pipe, the grains of the steel pipe are not refined and, therefore, it is impossible to sufficiently ensure the toughness of a steel pipe to be obtained.